The proposed research bridges the fields of neurobiology and biochemistry to provide the foundation for understanding common mechanisms causing trinucleotide repeat disorders including spinocerebellar ataxias, fragile X-associated tremor ataxia syndrome and Huntington's disease. This work will focus on the role of expanded CAG-repeat RNA, which has been shown to be toxic, leading to neural dysfunction and degeneration. A plan is proposed to determine how expanded repeat RNA is toxic. First, cell biological techniques will be used to determine the subcellular localization of the RNA in vivo (Aim 1). Colocalization of CAG-repeat RNA with markers for various subcellular structures will give valuable clues to the protein machineries that may be affected in response to RNA toxicity. Second, non-biased biochemical approaches will be used to identify cellular components present in brain tissues that interact with expanded trinucleotide repeat RNA (Aim 2). Third, Drosophila will be used as an experimental model system to define the contribution of these binding partners and orthologues in neurological disease (Aim 3). Genetic assays, using pre-existing mutant alleles or RNAi knockdown strains, will be performed to determine how the candidate gene products modulate trinucleotide repeat RNA toxicity in vivo. Combined, these efforts will provide the basis for understanding mechanisms of RNA toxicity in spinocerebellar ataxias, and advance the long-term goal of blocking or reversing disease progression. This work pioneers an area so far largely neglected in neurodegenerative disease, and holds the promise of opening up new grounds for drug targeting and therapeutical approaches. PUBLIC HEALTH RELEVANCE: There are more than 40 human trinucleotide repeat diseases which include various ataxias, Huntington's disease, muscular dystrophy and fragile X syndrome. The research proposed will provide greater understanding of molecular mechanisms that are common to several trinucleotide repeat disorders. These studies will lay the foundation for novel therapeutic approaches for treatment of these diseases.